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Fair Value of Financial Instruments - Summary of Composition of Short and Long Term Investments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 492,537 $ 448,492
Unrealized Gains 59 8
Unrealized Losses (385) (443)
Aggregate Fair Value 492,211 448,057
Commercial Paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 8,256 5,874
Unrealized Losses (14) (7)
Aggregate Fair Value 8,242 5,867
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 70,958 81,947
Unrealized Gains 3  
Unrealized Losses (233) (279)
Aggregate Fair Value 70,728 81,668
US Government Agency Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost   4,000
Unrealized Losses   (13)
Aggregate Fair Value   3,987
US Treasury Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 413,323 356,671
Unrealized Gains 56 8
Unrealized Losses (138) (144)
Aggregate Fair Value $ 413,241 $ 356,535